This invention relates generally to systems for producing a three-phase AC output voltage, and more particularly, to an arrangement which utilizes three free-running inverters for generating three-phase AC voltages from a DC voltage supply.
It is known in the art that a three-phase AC system can be formed from three single-phase inverters. In such known inverter arrangements, the single-phase inverters utilize control pulses which are received from a clock generator having an output frequency which is three times the frequency of the desired output AC voltage, the pulses being counted so as to produce the appropriate output phase. Alternatively, an inverter arrangement having a three-phase output can be constructed using a three-phase bridge circuit. One such inverter arrangement is described in Thyristor-Handbuch, by Hoffman-Stocker, 1965, page 252. The arrangement described therein utilizes low-pass filters to reduce harmonic content (see FIG. 169 of Hoffmann-Stocker).
It is a problem with low-pass filters of the type used to reduce the harmonic content of the AC output voltage of known three-phase inverter systems, that the amplitude and phase characteristics of the filter vary as a function of the load. This problem is compounded by the fact that the series inductor of a low-pass filter must be large so as to provide adequate short-circuit resistance to the inverter, but such a large inductance increases the magnitude of the variations in phase and amplitude, in response to variations in the magnitude of the load. Thus, if the load of a three-phase system is unbalanced, the series inductance of the low-pass filters will cause the output phases of the system to be asymmetrical with respect to one another, in terms of amplitude and phase.
It is, therefore, an object of this invention to provide an inverter arrangement in which a three-phase symmetrical output AC voltage is insured even though the load is unbalanced.